Underlying Event Models Mike Seymour ManchesterCERN Te V

Underlying Event Models Mike Seymour Manchester/CERN Te. V 4 LHC September 16 th 2004

Underlying Event Models 1. 2. 3. 4. What do mean by underlying event? Why are we interested? How do we model them? Where are we heading?

What do we mean by the Underlying Event? “Everything except the hard process” but… • initial state radiation • factorization scale • parton distribution functions • parton evolution underlying event model integral part of event model Underlying Events Mike Seymour 3

Why should we be interested? 1. QCD 2. Physics Experiments 3. Connection with: Occupancy Jet cross sections diffraction Pile-upreconstruction Mass saturation Backgrounds Rapidity gaps/jet vetoes confinement Emiss reconstruction total cross section Photon/lepton isolation Can we predict/understand the properties of hadrons? “Don’t worry, we will measure and subtract it” But… fluctuations and correlations crucial Underlying Events Mike Seymour 6

Fluctuations and correlations log Steep distribution ) small sideways shift = large vertical Rare fluctuations can have a huge influence ) corrections depend on physics process pt Underlying Events Mike Seymour 7

How do we model the Underlying Event? • Uncorrelated soft scatter – HERWIG/UA 5 model Parameterization of data • Broad multiplicity distributions ) large fluctuations • Long range correlations But… • Energy-dependence? • Hard component? • Hard/soft correlation? • Multiple interactions – hard and/or soft Hadron-hadron collision is incoherent sum of many parton collisions Underlying Events Mike Seymour 8

HERWIG’s Soft Underlying Event model G. Marchesini & B. R. Webber, PRD 38(1988)3419 Compare underlying event with ‘minimum bias’ collision Parameterization of (UA 5) data + model of energy-dependence Underlying Events Mike Seymour 9

Pedestal Effect http: //www. phys. ufl. edu/~rfield/cdf/chgjet_intro. html Underlying Events Mike Seymour 10

but suffers from lack of a hard component… http: //www. phys. ufl. edu/~rfield/cdf/chgjet_intro. html Underlying Events Mike Seymour 11

Multiple interactions – b-space picture ¼ instantaneous sampling of disk of partons b 1 b b 2 Only need to know f(x, b) and total parton-parton cross section ij Simplest possible model: f(x, b)=f(x)G(b) Eikonal approximation: multiple scatters are independent of each other Underlying Events Mike Seymour 12

Correlations and fluctuations • Simple b-space correlation Small cross section process has occurred ) bias to large overlap ) underlying event À minimum bias • More b-space correlation Expect high-momentum partons to be more central? • Energy-momentum conservation Backward evolution ) even soft scatters consume a lot of energy » 10% Scatters cannot be independent (Multiple scatters within evolution chain? Sjöstrand Skands) • Colour connections Each parton scattering hadronizes independently? Colour connections? Reconnections? Sjöstrand Skands Underlying Events Mike Seymour 13

Jimmy – Multiparton Interactions in HERWIG J. M. Butterworth, J. R. Forshaw & MHS, ZPC 72(1996)637 Starting point: eg (EM form factor) Underlying Events Mike Seymour 14

Multiparton Interactions Assume: n-parton distributions uncorrelated: Poisson distribution at fixed impact parameter Underlying Events Mike Seymour 15

Non-Poissonian Distribution ~ Geometric Distribution and b-space correlations (saturates at pt~10 Ge. V) Underlying Events Mike Seymour 16

Energy Conservation Narrows distribution. Ask for infinite number of extra scatters. How many do you get? Underlying Events Mike Seymour 17

+ Many other choices… • Scattering cross sections calculated with standard pdfs • But initial state shower/remnant model gluon only • Colour connections between scatters Underlying Events Mike Seymour 18

T. Sjöstrand P. Z. Skands Underlying Events (JHEP 0403: 053, 2004) Mike Seymour 19

Proton Radius parameter within Jimmy I. Borozan, Ph. D thesis, unpublished • Increasing 2 to 2 Ge. V 2 (i. e. decreasing proton radius by 40%) with ptmin=3 Ge. V gives ~ perfect description of Tevatron data… Transverse but, energy dependence? Need a model… Godbole? Underlying Events Mike Seymour 20

Proton Radius parameter in total cross section R. Godbole et al, hep-ph/0408355 • Starting point Jimmy+Ivan • Resum dynamical effects due to soft gluon emission Underlying Events Mike Seymour 21

Recent Progress • HERWIGv 6. 5 + Jimmy v 4. 1 (http: //hepwww. rl. ac. uk/theory/seymour/herwig/) – Plug and play add-on to HERWIG – Simulates underlying event in high-Et jet processes and any other hard process correctly (for first time!) • Sjöstrand Skands completely new model for multiple interactions in PYTHIA (JHEP 0403: 053, 2004 and hep-ph/0408302) – Includes completely new initial-state cascade model – Colour (re)connection within and between interactions – Multiple interactions within one parton-parton collision • Borozan and MHS model of multi- hard and soft scatters – Proof of principle for possible future model… Underlying Events Mike Seymour 22

Ivan – Multiple soft interactions in HERWIG I. Borozan & MHS, JHEP 0209(2002)015 • • • Partons still independent Soft scatters below PTMIN Gluon—gluon only: x g(x) = const ‘Gaussian’ distribution in pt Continuity at pt=PTMIN Take Eikonal seriously… No new free parameters! Underlying Events Mike Seymour 23

Jimmy doesn’t leave much room for Ivan! • For PTMIN<2 Ge. V, hard cross section saturates total… • Together with matching condition, inverts Gaussian… pt Underlying Events Mike Seymour 24

Similar to Jimmy with low PTMIN, but smaller PTMIN dependence Still needs study of radius dependence Jimmy+Ivan Jimmy PTMIN=2 Ge. V PTMIN=3 Ge. V Underlying Events Mike Seymour 25

To Do List (July 2003) • • Upgrade Jimmy to HERWIG 6. 5 This month PTMIN(HW) > PTMIN(Jimmy) Real soon now Robust distribution of Jimmy within HERWIG This year…? Robust distribution of Ivan within HERWIG++ …? Underlying Events Mike Seymour 26

Work in we Progress Where are heading? • Understanding Jimmy – small-x partons play a huge role • eg PTMIN=3 Ge. V @ LHC probes x» 10 -7 – proton radius parameter from Godbole et al model? • Understanding new PYTHIA – huge job! Need to tune • new initial state shower? • colour connection options? • matter distribution? • New models – SHERPA (Frank Krauss et al) – Jimmy+Ivan HERWIG++ – Multiple interactions in the LDC model (Leif Lönnblad) Underlying Events Mike Seymour 27

Summary • Underlying events are important for almost all physics measurements at LHC, extremely important for some • Theoretical activity is increasing » 4 new models available now/soon • HERA and Tevatron are excellent testing grounds • Don’t forget SPS data too • Deeper theoretical understanding needed Underlying Events Mike Seymour 28